A lithium battery in the related art uses a lithium metal as a negative active material, but when a lithium metal is used, a battery is short-circuited by formation of dendrite to cause danger of explosion, so that a carbon-based material is widely used as a negative active material, instead of a lithium metal.
The carbon-based active material includes crystalline carbon, such as graphite and synthetic graphite, and amorphous carbon, such as soft carbon and hard carbon. However, the amorphous carbon has a large capacity, but has a problem in large irreversibility during a charge/discharge process. Graphite is representatively used as the crystalline carbon, and has a theoretical limit capacity of 372 mAh/g, and may be used as a negative active material.
However, even though a theoretical capacity of the graphite or the carbon-based active material is slightly large, the theoretical capacity is simply about 380 mAh/g, so that there is a problem in that the aforementioned negative electrode cannot be used when a large capacity lithium battery is future developed.
In order to solve the problem, research on a metal-based or intermetallic compound-based negative active material has been currently and actively conducted. For example, research on a lithium battery utilizing metal, such as aluminum, germanium, silicon, tin, zinc, and lead, or semimetal as a negative active material has been conducted. The material has a large capacity and a high energy density, and is capable of occluding and discharging larger lithium ions than the negative active material using the carbon-based material, so that it is possible to manufacture a battery having a large capacity and a high energy density. For example, it is known that pure silicon has a large theoretical capacity of 4,017 mAh/g.
However, compared to the carbon-based material, the metal-based or intermetallic compound-based negative active material has a cycle characteristic degradation to be obstacles to commercialization. The reason is that when the silicon is used as a negative active material for occluding and discharging lithium as it is, conductivity between active materials may deteriorate due to a change in a volume during a charge/discharge process, or a negative active material is peeled from a negative current collector. That is, the silicon included in the negative active material occludes lithium by charging and is expanded to have a volume of about 300 to 400%, and when lithium is discharged during the discharge, mineral particles are contracted.
When the aforementioned charge/discharge cycle is repeated, electric insulation may be incurred due to a crack of the negative active material, so that a lifespan of the lithium battery is sharply decreased. Accordingly, the aforementioned metal-based negative active material has a problem to be used in the lithium battery.
In order to solve the aforementioned problem, research on a negative active material having a buffering effect against a volume change by using particles having a nano size level as silicon particles or giving porosity to silicon is conducted.
Korean Patent Application Laid-Open No. 2004-0063802 relates to “Negative Active Material for Lithium Secondary Battery, Method of Manufacturing the Same, and Lithium Secondary Battery”, and adopts a method of alloying silicon and another metal, such as nickel, and then eluting the metal, and Korean Patent Application Laid-Open No. 2004-0082876 relates to “Method of Manufacturing Porous Silicon and Nano-size Silicon Particle, and Application of Porous Silicon and Nano-size Silicon Particle as Negative Electrode Material for Lithium Secondary Battery”, and discloses technology of mixing alkali metal or alkali earth metal in a powder state with silicon precursor, such as silicon dioxide, performing heat treatment on a mixture, and eluting the mixture as acid.
The patent applications may improve an initial capacity maintenance rate by a buffering effect according to a porous structure, but simply use porous silicon particles having conductivity deterioration, so that when the particles do not have a nano size, conductivity between the particles is degraded while manufacturing an electrode, thereby causing a problem of deterioration of initial efficiency or a capacity maintenance characteristic.